Radiation hardness test of preamplifier circuits composed of commercial bipolar transistors

In general, radiation-hardened transistors are very expensive and it is sometimes not easy to acquire proper ones for application. In this study, we designed a front-end electronic circuit for high-rate neutron counters installed in a high-level γ-radiation hot cell. All the transistors adopted for this circuit are not radiation-hardened ones, i.e. commercial ones not specifically processed for a radiation resistance. The aim of our study was to seek more radiation-resistant transistors from among the commercial ones and to verify the radiation hardness of the circuit composed of these transistors. The circuit includes a preamplifier, a comparator, and a monostable multivibrator. To realize the radiation hardness of this circuit with commercial transistors, the transistors were categorized into two groups: general speed and high-speed transistors. After a 100 Mrad irradiation from a 60Co γ-ray source, the reduction of the current gain of the general speed transistors was over 80% and that of the high-speed transistors was below 68%. The signal-to-noise (S/N) ratio of the preamplifier output voltage was reduced by 66% for the former and by 36% for the latter.